The US is heading toward a catastrophic — and avoidable — default. Here's what the debt ceiling is, and why it's so dangerous.

• Congress is sparring again over raising the debt ceiling, and time is running out to avoid a default.
• The ceiling caps how much the government can borrow, but it's turned into a political weapon.
• Here's what the debt ceiling is and why it's so dangerous for the US economy.
• See more stories on Insider's business page.

The clock is ticking toward a catastrophic, and unprecedented, default on the nation's debt, and Congress isn't doing anything about it.